(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

September 14, 2011

(Date of Event which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), Rule
13d-1(f) or Rule 13d-1(g), check the following box. ¨

Note: Schedules filed in
paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7(b) for other parties to whom copies are to be sent.

*

The remainder of this cover page shall be filled out for a reporting persons initial filing on this form with respect to the subject class of securities, and for
any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information
required on the remainder of this cover page shall not be deemed to be filed for the purpose of Section 18 of the Securities Exchange Act of 1934 (Act) or otherwise subject to the liabilities of that section of the Act but
shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 458786100

SCHEDULE 13D/A

Page
2
of 5

1.

Names of Reporting Persons.

I.R.S.
Identification Nos. of Above Persons (entities only).

Kemper
Corporation

95-4255452

2.

Check the Appropriate Box If a
Member of a Group (See Instructions)

(a) ¨ (b) ¨

3.

SEC Use Only

4.

Source of Funds (See
Instructions)

N/A

5.

Check If Disclosure of Legal
Proceedings Is Required Pursuant to Items 2(d) or 2(e) ¨

Item 4 of
the Schedule 13D is hereby amended and supplemented by adding the following paragraph at the end of Item 4:

Trinity sold 250,000 shares
of Common Stock in open market transactions following September 2, 2011, the date of the last sale reported in Amendment No. 16 (the most recently filed amendment to the Schedule 13D), through September 12, 2011. On September 14,
2011, Trinity transferred all of its remaining shares of Common Stock, or 7,309,764 shares, to its parent company, Kemper Corporation, which was an indirect beneficial owner of the shares prior to the transfer. Also on September 14, 2011,
Kemper Corporation contributed all of the 7,309,764 shares of Common Stock to the Kemper Corporation Master Retirement Trust.

ITEM 5.

Interest in Securities of the Issuer.

Item 5 of the Schedule 13D is hereby amended and restated in its entirety as follows:

As indicated on the cover pages, as of September 14, 2011, Kemper and Trinity are no longer beneficial owners of any shares of Common Stock. Trinity sold 250,000 shares of Common Stock in open market
transactions following September 2, 2011, the date of the last sale reported in Amendment No. 16 (the most recently filed amendment to the Schedule 13D), through September 12, 2011. The following is a listing of these sales:

Transaction Date

Number of Shares Sold

Price per Share*

9/6/2011

50,000

6.6557

9/7/2011

50,000

7.0336

9/8/2011

50,000

6.9948

9/9/2011

50,000

6.6309

9/12/2011

50,000

6.6286

*

Represents the weighted average sale price per share of the shares sold on the dates specified. All sales were reported on Form 4 reports filed pursuant to
Section 16 of the Securities Exchange Act of 1934. The Form 4 reports contain additional detail on the sale prices of the shares sold on each date.

On September 14, 2011, Trinity transferred all of its remaining shares of Common Stock, or 7,309,764 shares, to its parent company, Kemper Corporation, which was an indirect beneficial owner of the
shares prior to the transfer. Also on September 14, 2011, Kemper Corporation contributed all of the 7,309,764 shares of Common Stock to the Kemper Corporation Master Retirement Trust.

Except as otherwise described in this Amendment No. 17 to Schedule 13D, to the best knowledge and belief of the filing persons: (i) during the past sixty (60) days, no filing person nor
anyone listed in the Schedules to this Schedule 13D has entered into any transactions involving the Common Stock; and (ii) no one listed on the Schedules to this Schedule 13D beneficially owns shares of Common Stock.

CUSIP No. 458786100

SCHEDULE 13D/A

Page
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of 5

SIGNATURE

After reasonable inquiry and to the best of the undersigneds knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

DATED: September 16, 2011

KEMPER CORPORATION

By: /s/ JOHN BOSCHELLI

Name: John M. Boschelli

Title: Vice President and Chief Investment Officer

TRINITY UNIVERSAL INSURANCE COMPANY

By: /s/ JOHN BOSCHELLI

Name: John M. Boschelli

Title: Assistant Treasurer

The original statement shall be signed by each person on whose behalf the statement is filed or his authorized
representative. If the statement is signed on behalf of a person by his authorized representative (other than an executive officer or general partner of the filing person), evidence of the representatives authority to sign on behalf of such
person shall be filed with the statement; provided, however, that a power of attorney for this purpose which is already on file with the Commission may be incorporated by reference. The name and any title of each person who signs the
statement shall be typed or printed beneath his signature.